In conventional machine tools requiring a high degree of precision, it is conventional to employ precision ground lead screw assemblies. Such a lead screw assembly typically includes a lead screw into which a recessed, spiral-shaped race has been ground, and a nut in which an array of ball bearings has been packed. The ball bearings and the nut are sized so that the ball bearings ride in the race when the nut mates with the screw. As the screw advances relative to the nut, the ball bearings roll along the race. Using such an assembly, the machine tool operator can accurately control the position of the lead screw, typically to within less than 10.sup.-3 inch
In other types of conventional linear bearing assemblies, ball bearings (which are sometimes not mounted within a nut) roll in a race (which is sometimes not spiral-shaped) between a first component and a second component, as the first and second components translate linearly with respect to each other.
In constructing a linear bearing assembly, variations in ball bearing size, surface condition, pitch, and other ball bearing characteristics typically cannot be avoided. Such variations can undesirably increase the torque necessary to translate the assembly's first and second components relative to each other, and the friction and tracking error associated with the assembly.
It is often desirable to design a linear bearing assembly (such as a lead screw assembly) to have a very low starting torque (i.e., very low rolling resistance when a load is first applied to the ball bearings) in addition to a high running efficiency, particularly when the assembly is to be used in low heat environments such as a high vacuum.
However, until the present invention, it had not been known how to design a linear bearing assembly to have both extremely low starting torque and high running efficiency, while also avoiding performance degradation resulting from inherent manufacturing variations in the size, surface condition, and other characteristics of the ball bearings employed.